Colony Morphology

Dr. Md. Abdus Shabur TalukderPhD (Japan)

•Bacteria grow as colonies on solid media. A colony is a visible mas of

microorganism that originated from a single mother cell. Hence, a colony of bacteria

is a clone of genetically alike bacteria.

•There are various types of bacteria and each type produces differently looking

colonies. They vary in color, share, pigmentation, and other characteristics.

•Colony morphology is a way of identifying bacteria. By observing the colony of

bacteria, the identity of bacteria will be determined.

Colonial morphology

Colonial morphologyIn microbiology, colonial morphology refers to the visual

appearance of bacterial or fungal colonies on an agar

plate. Examining colonial morphology is the first step in the

identification of an unknown microbe. The systematic

assessment of the colonies appearance, focusing on

aspects like size, shape, colour, opacity, and

consistency, provides clues to the identity of the

organism, allowing microbiologists to select appropriate

tests to provide a definitive identification.

The image shows the colony morphology of bacteria.

Procedure

•When a specimen arrives in the microbiology laboratory, it is

inoculated into an agar plate and placed in an incubator to

encourage microbial growth. Because the appearance of

microbial colonies changes as they grow, colonial

morphology is examined at a specific time after the plate is

inoculated. Usually, the plate is read at 18–24 hours post-

inoculation, but times may differ for slower-growing organisms

like fungi. The microbiologist examines the appearance of the

colony, noting specific features such as size, colour, shape,

consistency, and opacity. A hand lens or magnifying glass

may be used to view colonies in greater detail.

• The opacity of a microbial colony can be described as transparent,

translucent, or opaque. Staphylococci are usually opaque, while many

Streptococcus species are translucent. The overall shape of the colony may be

characterized as circular, irregular, or punctiform (like pinpoints). The vertical

growth or elevation of the colony, another identifying characteristic, is assessed by

tilting the agar plate to the side and is denoted as flat, raised, convex, pulvinate

(very convex), umbilicate (having a depression in the centre) or umbonate

(having a bump in the centre). The edge of the colony may be separately

described using terms like smooth, rough, irregular and filamentous. Bacillus

anthracis is notable for its filamentous appearance, which is sometimes described

as resembling Medusa's head.

•Consistency is examined by physically manipulating the colony with a sterile

instrument. It is described using terms like brittle, creamy, sticky and dry.

Staphylococci are considered to have a creamy consistency, while some

Neisseria species are sticky, and colonies of diphtheroid bacteria and beta-

hemolytic streptococci are typically dry. Bacteria that produce capsules often

have a slimy (mucoid) consistency.

•When certain microorganisms are grown on blood agar, they may digest the blood

in the medium, causing visible hemolysis (destruction of red blood cells) on the

agar plate. In colonial morphology, hemolysis is classified into three types:

•alpha

•beta

•and gamma-hemolysis.

•In alpha-hemolysis, the blood is partially digested, causing the area around the

colony to turn green. In beta-hemolysis, the organism digests the blood completely,

leaving a clear area around each colony. Organisms that do not produce

hemolysis are referred to as gamma-hemolytic. Clostridium perfringens, which

causes gas gangrene, is noteworthy for producing a "double zone" of both

complete and incomplete hemolysis.

•The odour of a culture is sometimes considered part of colonial morphology.

While intentionally smelling microbial cultures is not advised, some organisms

produce distinctive odours that can be detected during routine examination of the

culture. Among these are Pseudomonas aeruginosa, which has a grape-like

scent; Staphylococcus aureus, which is said to smell like old socks; and

Proteus mirabilis, whose scent is alternately described as putrid or like

chocolate cake.

•Other distinctive features of colonial morphology include motility and the

production of pigments. Pseudomonas aeruginosa produces the

pigments pyocyanin and pyoverdin, which give the colonies a greenish sheen.

Some specimens of Serratia marcescens produce an orange-red pigment called

prodigiosin. Organisms with swarming motility, like Proteus species, exhibit

concentric waves of growth extending from the inoculation point.

Images of the bacterial colony with a varying degree of pigmentation

Staphylococcus aureus: large opaque, round, creamy, white to yellowish

colonies displaying beta-hemolysis on blood agar

Streptococcus pyogenes: small translucent colonies displaying beta-

hemolysis on blood agar

Streptococcus pneumoniae: small colonies with raised edges displaying alpha-hemolysis on blood agar

Proteus sp.: swarming behavior on blood agar

Serratia marcescens: red pigmentation: although considered characteristic of the

species, only about 10% of specimens produce this pigment

Bacillus cereus: "ground-glass" colonies displaying beta-hemolysis on blood agar

Aspergillus niger: granular colonies with a white edge and central black pigmentation

Yeasts – The colony of

yeast, which is a type of

fungi, is somewhat similar to

that of the colony of bacteria.

They can grow as a white

patch with a glossy surface.

A petri dish containing mold growth

Factors affecting the colony morphology of bacteria

•Type of media – The cultural characteristics of bacteria can be affected by the type of

media and the nutrient it contains. Keep in mind that some types of media are more

nutritive than others. The more nutritive the media is the more it encourages hearty

growth. On the other hand, some types of media can restrict growth.

•Temperature – Some bacteria grow more rapidly at body temperature while others are

weak at room temperature. More so, some bacteria form pigment under favorable

temperature.

•Length of time – The incubation time/period may affect the colonial characteristics and

colonial size of bacteria.

•Presence of other organisms – The growth of bacteria can be affected by the presence

of other organisms.

Interpretation

•Colonial morphology serves as the first step in the identification of microbial

species from clinical samples. Based on the visual appearance of the colonies,

microbiologists can narrow down the list of possible organisms, allowing them to

select appropriate tests to provide a definitive diagnosis. For example, if a

microbiologist observes colonies that resemble a Staphylococcus species, they

may perform a catalase test to confirm that it belongs to the genus

Staphylococcus, and a coagulase test to determine whether it is a coagulase-

negative staphylococcus or a more pathogenic species, such as S. aureus.

•Observation of hemolysis is useful in the identification of bacteria, especially

streptococci, which are classified on the basis of their hemolytic reactions. For

example, Streptococcus pyogenes, which causes strep throat and scarlet fever,

displays beta-hemolysis, while Streptococcus pneumoniae, which can cause

pneumonia and meningitis, displays alpha-hemolysis. The highly pathogenic S.

aureus classically displays beta-hemolysis, while Staphylococcus epidermidis, part

of the normal skin flora and an occasional opportunistic pathogen, does so weakly

or not at all.

•Although automated techniques like MALDI-TOF (Matrix-assisted laser

desorption/ionization time-of-flight (MALDI-TOF) are increasingly used to identify

microorganisms in clinical laboratories, colonial morphology remains useful to

distinguish potential pathogens, which must be identified, from normal flora, for

which definitive identification is unnecessary, and to confirm identification when

automated techniques give inconclusive results.